In a wire bonding process, electrically conductive wires are bonded between electrical bonding pads found on semiconductor devices, such as between a semiconductor die and a substrate onto which the die is attached. The substrate is usually a semiconductor leadframe. The electrical connection could also be made between bonding pads found on separate semiconductor dice. The bond is formed by a bonding tool which may be in the form of a capillary attached to an ultrasonic transducer for generating ultrasonic energy to the capillary tip.
Wire is continually fed to the capillary in order to make wire connections. The wire feeding system may comprise a clamp that is operative to secure the wire relative to the bonding tool in order to pull and feed a length of wire to the capillary, as well as a wire tensioner to pull the wire in a reverse direction. The need to pull the wire in a reverse direction may occur, for instance, after a molten free-air ball is formed at a tail end of the wire after sparking with an electronic flame-off (EFO) wand, and when forming wire loops thereafter.
The wire bonding apparatus commonly uses a wire tensioner actuated by air to ensure the centering of the bonding wire and the molten ball with respect to the capillary tip before ball bonding is performed. This is to ensure accuracy of placement of the bonded ball at the first bond. After EFO sparking, the formed ball is pulled up by the wire tensioner to sit in a central position under the capillary. The consistency of ball centering relies on the stability of the pulling force exerted by the wire tensioner. Furthermore, consistency of the wire tension force is necessary to promote wire looping consistency, so that the loop heights of multiple wire loops that are formed can be better controlled. Therefore, periodic checking and cleaning of the wire tensioner is required to ensure the consistency of ball centering and of the wire tensioning force.
Conventionally, the wire is led through a bore of the wire tensioner, and the wire may be pulled by a force generated when compressed air is injected at high pressure into the bore to pull the wire. A prior art wire tensioner that utilizes compressed air is described in US Patent Publication No. 2006/0091181A1 entitled “Wire Tensioner for a Wire Bonder”. The wire tensioner that is described has a body structure defining a passage for receiving a wire, the passage including an inlet opening and an outlet opening through which the wire is configured to extend. The wire tensioner has an inlet port through which pressurized fluid is received into the wire tensioner and an exhaust port through which pressurized fluid is exhausted from the wire tensioner.
Use of a compressed air to generated the aforesaid pulling force has the disadvantage that, on its own, a high volume of compressed is required to generate a sufficient force to move the wire. As a result, the problem of turbulence created in the air flow as it exits from the tensioner causes undesirable wire vibration and spinning that may impose torque and whipping disturbances to the wire being fed through the wire tensioner. Furthermore, the prior art design generates compressed air directly into the passage where the wire is located, which further aggravates the problems of wire vibration and spinning when compressed air is injected directly onto the wire.
Another approach for generating the necessary wire tensioning force in a wire tensioner is by the use of vacuum suction force. FIG. 1 is a schematic cross-sectional view of a conventional vacuum wire tensioner 100 that is available in the prior art. The wire tensioner 100 comprises a wire inlet 102, inner tube 104 and wire outlet 106 having a central bore 108 extending through the respective wire inlet 102, inner tube 104 and wire outlet 106. A bonding wire 110, such as gold wire, is led through the central bore 108 from the wire inlet 102 to the wire outlet 106 whereat the bonding wire 110 is extended towards a capillary (not shown). The wire inlet 102, inner tube 104 and wire outlet 106 are substantially enclosed by a wire tensioner housing 112, which also serves as a means for securing the wire tensioner 100 to a wire bonding apparatus.
Near a mid-point of the wire tensioner housing 112, a vacuum port 114 is located to generate a vacuum suction force at the base of the inner tube 104. This vacuum suction force is operative to pull the bonding wire 110 located in the wire outlet 106 upwards. In order to supplement the vacuum suction force from the vacuum port 114, a compressed air inlet 116 is located at the top of the inner tube 104 to generate a force to push the bonding wire 110 upwards. To ensure that a minimum of air passes through the inner tube 104 and in order to facilitate the creation of the above air flows through the wire inlet 102 and wire outlet 106, the central bore 108 in the inner tube 104 is formed with a smaller cross-sectional area than the central bores 108 of the wire inlet 102 and the wire outlet 106. The combination of forces from the vacuum port 114 and compressed air inlet 116 generate a sufficient force to reliably pull the wire without causing any significant wire vibration and spinning encountered by using compressed air alone.
However, a problem with the conventional design illustrated in FIG. 1 is that ambient air 118 containing contaminants generated from the surrounding during wire bonding are sucked into the central bore 108 of the wire tensioner 100 through the mouth of the wire outlet 106. After a period of usage, the inner components of the wire tensioner may become clogged with contaminants, and this results in the necessity for frequent cleaning of the wire tensioner 100. Frequent cleaning is inconvenient to the operator and leads to down-time of the wire bonding apparatus.